1. Field of the Invention
The present invention relates to a color cathode ray tube and, more particularly, to an improvement in a shadow mask for use in a color cathode ray tube.
2. Description of the Related Art
The shadow mask of a color cathode ray tube has a large number of apertures. These apertures are so designed as to have a geometrical one-to-one correspondence with phosphor layers. Each aperture thus designed passes an electron beam emitted from an electron gun, the passed electron beams each being incident on a corresponding phosphor layer which is in a geometrical one-to-one correspondence with that aperture. Therefore, the aperture is also called a color selecting electrode.
In a typical color cathode ray tube, about 15% to 20% of an entire electron beam emitted from an electron gun reaches phosphor screen through the apertures of a shadow mask, and the remaining 80% to 85% portion of the beam is incident on the surface of the shadow mask. As a result, the kinetic energy of the electron beam is converted into a thermal energy that heats the shadow mask to about 80.degree. C. Generally, the base material used in the shadow mask is a cold-rolled iron plate 0.1 to 0.3 mm in thickness whose thermal expansion coefficient is 12.times.10.sup.-6 /.degree.C. at 20.degree. C. to 100.degree. C. When heated as described above, the shadow mask causes thermal expansion generally referred to as doming. This thermal expansion brings about a geometric positional deviation between the apertures of the shadow mask and a phosphor layer. Consequently, a portion of an electron beam passing through the apertures is incident on a phosphor layer of another color, leading to a purity drift.
To improve a color cathode ray tube which causes a significant purity drift due to the doming phenomenon, Jpn. Pat. Appln. KOKOKU Publication No. 42-25446, e.g., has proposed the use of an iron-nickel alloy, such as an invar alloy, whose thermal expansion coefficient is nearly 1/10 that of iron. Unfortunately, the invar alloy is expensive, having a high yield strength after annealing and a low yield in mask molding. Therefore, color cathode ray tubes using such an invar alloy are very expensive compared to those using iron.
For this reason, a coating has conventionally been formed on the surface of a shadow mask for suppressing a purity drift caused by the doming
The above-mentioned prior art will be further described below.
In a first method, as proposed in Jpn. Pat. Appln. KOKAI Publication No. 60-54139, crystallized glass formed from lead borate is coated on the surface of a shadow mask and bonded by a high-temperature heat treatment to suppress doming. This method incorporates lead, a harmful substance, into the glass layer. Therefore, extraordinary safety measures must be undertaken in handling the material to ensure a safe working environment and to prevent an environmental pollution.
A second method uses a coating solution containing particles of a heavy metal substance whose atomic number exceeds 70, as proposed in Jpn. Pat. Appln. KOKOKU Publication No. 60-14459. In this method, the coating solution is spray-coated on the surface on the electron beam incident side of a shadow mask to form a coating having an electron beam reflecting property. In this method, a water-soluble suspension containing fine particles of a heavy metal, such as bismuth oxide, must also be sprayed on the electron beam incident surface of the shadow mask after the formation of the coating. Therefore, the effectiveness of this method in preventing the purity drift resulting from the doming of the shadow mask depends solely upon a single element, such as bismuth oxide. The effectiveness of this method is therefore unsatisfactory when compared with the effectiveness of a shadow mask having no electron beam reflecting coating.
A third method suppresses doming by increasing the thermal conductivity or thermal radiation efficiency of the shadow mask, while imparting the electron beam reflecting property discussed above. As a method of this sort, Jpn. Pat. Appln. KOKAI Publication No. 4-48530 describes a method of forming a solution by mixing bismuth oxide particles, tungsten particles, and partially graphitized carbon particles with water glass, and forming a composite coating on the electron beam incident surface of the shadow mask by coating the solution on a shadow mask Through this method, the purity drift preventing effect is relatively good. However, since the particle sizes of the raw materials are large, it is difficult to uniformly mill the materials even if the materials are milled and stirred to have an average particle size of, e.g., approximately 2 .mu.m by using a ball mill or the like. Consequently, it is difficult to obtain a sharp particle size distribution of the milled particles. In order to prevent deformation or clogging of the mask apertures, the thickness of the coating must be controlled to about 3 .mu.m. However, since substances having no sharp particle size distributions and different specific gravities are mixed, it is impossible to obtain a homogeneous mixture as the coating solution. This inhomogeneous coating solution cannot be spray-coated, rendering it is difficult to perfectly coat a shadow mask with this coating solution.
A fourth method is disclosed in Jpn. Pat. Appln. KOKAI Publication NO. 62-110240. In this method, an amorphous metal oxide material or the like is used as a binder to form a layer containing a metal with a small atomic number, thereby improving the thermal radiation efficiency. In addition, a purity drift is prevented by performing electrostatic correction for the electron beam path by electrification.
As described above, a coating has been formed on the surface of a shadow mask using various methods to suppress the doming of the shadow mask. Because the coating used in each method is formed from a substance which does not melt at temperatures applied during the color cathode ray tube manufacturing process, water glass or a metal alkoxide must be used as a binder to allow film formation.
Since, however, water glass contains an alkali metal, a carbonate is also readily formed. This carbonate produces carbonic acid gas in a heat treatment during the manufacture, and a portion of the gas is readily adsorbed in the tube. The carbon dioxide adsorbed is released to poison the cathode an electron beam is incident upon the cathode ray tube while in operation. When the carbon dioxide is released the emission characteristics are degraded. Furthermore, a metal alkoxide does not form a perfect metal oxide when subject to a heat treatment of about 500.degree. C. Due to these imperfections, hydrogen gas is produced during operation of the color cathode ray tube. The product ionized hydrogen gas impinges causes ion burn on the phosphor screen,resulting in a decreased luminance.